1. Field of the Invention
The present invention relates to a method for locating inaccessible objects, particularly, but not exclusively, for locating underground boring tools.
2. Summary of the Prior Art
When locating underground objects it is known to arrange for a magnetic field to be generated by the object or by a field source attached thereto. In the case of underground conductors such as cables or pipes, an alternating current can be applied to the conductor to induce a cylindrical magnetic field with a detectable field strength at ground level. By measuring the variation of the field strength at the surface, the path of the underground conductor can be determined.
The locating of objects such as sub-surface boring tools cannot normally be performed in this way because a cylindrical field cannot be generated. Instead a magnetic field source is mounted on the boring tool and the field from that field source is detected. This field source can be a solenoid. When alternating current flows through the solenoid a bipolar magnetic field is generated which can be located at the surface by a person with a hand-held detector. The vertical component of the field at the surface will change direction when the field detector is directly above the solenoid, assuming the solenoid is horizontal. Therefore by noting the position in which that component of the field reverses the position of the solenoid in a horizontal plane can be determined. If this is done continuously, the movement of the boring tool on which the solenoid is mounted can be tracked. The depth of the solenoid can also be gauged by measuring the attenuation of the field at the surface. Of course, this requires the field strength at the solenoid to be known.
In WO 96/29615, we described the location of an inaccessible object by detecting the magnetic field generated by a solenoid at a remote site, making use of the relationship between the ratio of the axial and radial magnetic fields at any point in the plane containing the axis of the solenoid and the angle between the axis of the solenoid and the line joining the point to the centre of the solenoid. Where the orientation of the solenoid may change, as in an underground boring tool, it was then necessary for the boring tool to incorporate a tilt sensor to determine the orientation of the solenoid relative to the horizontal. This enabled the axial and radial fields to be determined from the fields measured.